This invention relates in general to operational amplifiers and, more specifically, to a rail-to-rail input stage of an operational amplifier.
Operational amplifiers are found in a myriad of applications in electronic circuit design for amplifying and/or buffering an input signal and providing an output signal. One desirable feature of an operational amplifier is the ability to drive an input signal between the positive and negative power supply rails (V.sub.CC and V.sub.EE). This feature is especially important when configuring the operational amplifier as a voltage follower with the output hard-wired back to the inverting input terminal while the input signal drives the non-inverting input terminal With the output signal of the operational amplifier swinging rail-to-rail, the input terminals must accommodate the same dynamic range.
Most operational amplifiers are limited in the dynamic range of the input signal, and the output signal, to a value somewhat less than the power supply rails, say 800 millivolts (mv) less. In applications involving relatively low operating potentials on the power supply conductors, say three volts between V.sub.CC and V.sub.EE, the 800 mv overhead limits the operating range of the input signal to 1.4 volts (3.0-0.8-0.8). A similar narrow operating range is a problem for the output signal of the operational amplifier. Such a narrow dynamic range is unacceptable in many applications.
Hence, what is needed is an operational amplifier having an input stage and an output stage each capable of processing signals operating substantially at the power supply rails.